Recommended HW for robust Linux+IO

[JorgeusMax]

The other problem is they aren't CE marked for industrial use.

That is true. The day a company produces a lowcost raspberry-like CE product will be a great day!

The best is then that I host the website and only install a 4G + low-cost PLC on site.
I can set up a VPN and communicate with the PLC via whatever protocol. Does anyone know which protocol alternatives I have? or do I have to stick with MODBUS?. Conventional sockets, although versatile, I am not aware that can be implemented in S7-1200 for example

 

[JesperMP]

Kunbus and the Onlogic device have the CE mark and EU Declaration of Conformity for Industrial use.
If you go for the Kunbus with Codesys, you also get protocols for PROFINET, EtherCAT, EtherNet/IP or CANopen. I suspect it will cost an extra license for these protocols.